Conventional implantable medical devices often obtain a rough measure of physiological parameters by sensing variation in voltage or other electrical characteristics along a single electrical path, field, or vector. For example, as shown in FIG. 1, such may be used to acquire rough impedance values that allow the implantable device to track a patient's respirations, since thoracic impedance varies with breathing. Energy transferred between a tip electrode and a case electrode typically provides a general-purpose electrical pathway or vector 100 consisting of electrons traveling pathways of least resistance or least impedance. Often, this technique produces some useful data in relation to the degree that the electrical vector achieved approximates a relevant vector orientation for measuring the target parameter. Tracking a physiological parameter is often correlated to a specific condition, such as respiration rate.
Congestive heart failure (CHF) or just “heart failure,” is a condition in which a patient's heart works less efficiently than it should, and in which the heart pumps insufficiently thereby depriving the body of the oxygen-carrying blood it requires, either during exercise or at rest. To compensate for this condition and to maintain cardiac output, the body retains sodium and water such that there is a build-up of fluid hydrostatic pressure in the pulmonary blood vessels that drain the lungs. As this hydrostatic pressure overwhelms oncotic pressure and lymph flow, fluid transudates from the pulmonary veins into the pulmonary interstitial spaces, and eventually into the alveolar air spaces. This complication of CHF is called pulmonary edema (PE), which can cause hypoxemia, respiratory acidosis, respiratory arrest, and death.
Similarly, CHF often involves dangerous decreases in chamber ejection fractions and enlargement of the ventricles and atria. Left atrial (LA) enlargement can result in atrial fibrillation. Left ventricular (LV) enlargement can result in ventricular tachycardias. Typically, known as ventricular dyssynchrony, the normal “V-V” delay between right ventricular contraction and left ventricular contraction is unduly lengthened during heart failure, further reducing pumping efficiency. Additionally, delays may appear between the electrical stimulation provided during the QRS complex of the cardiac cycle and the timing of the mechanical contraction that results from this electrical stimulation. These conditions typify the many adverse effects of heart failure that cause a viscous circle of failure: reduced efficiency begets attempts by the heart to compensate for the loss, but the compensation ultimately leads to further loss of efficiency, and so on. Therapy seeks to intervene before a point of no return is reached in the viscous circle.
At times the conventional implantable medical devices introduced above, in trying to track physiological parameters, can produce inaccurate data, false positives, and false negatives with respect to pathophysiological conditions like CHF. A false positive indicates the presence of a condition that does not really exist—because the conventional technique measures a parameter that is partly or entirely different than the target parameter, i.e., the specificity of the impedance measurement with regard to the parameter being measured is low. Conversely, a false negative indicates the absence of a condition that really is present—again, because the conventional technique measures a parameter that is different than that intended, or, because sensitivity to the parameter being measured is low.
Often conventional techniques examine a pathway of tissue in the body that is too limited or too unrelated to the parameter being measured—the parameters or conditions being measured are partly beyond the scope of the electrical pathway being utilized. The pathway may be limited because it is poorly selected with regard to the parameter sought, or because the same pathway is always used to measure many different kinds of diverse parameters, even parameters that are only tangentially related to the pathway. Using such a single general purpose vector has its limits and can even return misleading information, as just described.